Metal caps have been used widely for sealing containers, especially those used to hold carbonated beverages, because the metal caps were economical and easy to produce. A further advantage of the metal caps is that they provided high sealing performance when used together with a lining material.
In addition to closures which obtain a seal through the use of an independent liner such as is shown in U.S. Pat. No. 4,462,502, for example, it has been suggested to employ linerless closures which have integrally formed sealing elements. See generally U.S. Pat. Nos. 3,038,624; 3,568,871; 3,784,041; 3,815,771; 4,091,948; and 4,450,973.
Currently the industry is progressing towards the use of plastic caps because the consumer views the plastic caps as being more aesthetically pleasing. The major drawback with the use of plastic caps is their inability to retain a gas-tight seal after a period of time due to the material's creep characteristics. This becomes especially troublesome when the plastic caps are used to seal a carbonated beverage product.
The market has been flooded with a number of plastic caps of various designs which have attempted to properly seal the container but with only marginal success. U.S. Pat. Nos. 3,784,041 and 3,865,263, both issued to Birch, show various approaches for sealing a container with the use of an unlined plastic cap. The drawbacks with the Birch designs, wherein only a top seal is present, is that the seal tends to deteriorate over a period of time as the pressure of the carbonated beverage exerts upward pressure on the closure cap.
In spite of these prior art teachings, there remains therefore a very real and substantial need for a plastic closure which will provide an effective seal even when used with pressurized containers.